1. Field of the Invention
The present invention relates to a liquid-phase growth method for polycrystalline semiconductors such as silicon or the like which can be suitable used as semiconductors for solar cells, and to a liquid-phase growth apparatus suitable for carrying out this method.
2. Description of the Related Art
Increased public awareness regarding the environment has led to widespread consumer-grade use of solar cells. Consumer-grade solar cells primarily use monocrystalline or polycrystalline silicon wafers. Conventionally, monocrystalline silicon wafers have often been manufactured reusing silicon wafers which did not meet standards in the integrated circuit industry or the like, or silicone wafers left over from pulling, so there has been a limit in the amount thereof which can be supplied.
As for polycrystalline silicon wafers on the other hand, the throughput of the crystallization process is higher than that of mono crystals, but there still is a limit in the amount thereof which can be supplied in the event of using high-purity silicon for semiconductors as the ingredient thereof, and the cost cannot be reduced very much, either.
Now, methods for growing a polycrystalline silicon film on a ceramic substrate such as glassy carbon or mullite or the like are being attempted, but these normally require growth to be performed under high temperatures of 1,000 to 1,500° C., so inexpensive materials such as metal plate or glass or the like cannot be easily used, from the perspective of heat-resistance and matching of thermal expansion. Further, polycrystalline silicon film thus grown on such a substrate has small crystal grains and the smoothness of the surface tends to be poor, and accordingly practical use has yet to be achieved.
As an attempt for solving this problem, there is a method wherein polycrystalline silicon (metallurgical grade silicon) manufactured using inexpensive unrefined silicon is used as a substrate, and polycrystalline silicon grown thereupon using high-purity silicon is used as an activation layer, thereby forming a solar cell.
For example, Haruo ITO, Tadashi SAITOH, Noboru NAKAMURA, Sunao MATSUBARA, Terunori WARABISAKO, and Takashi TOKUYAMA have produced a trial solar cell by growing silicon poly crystals on a metallurgical grade silicon substrate by CVD (chemical vapor deposition) using SiH2Cl2 (J. Crys. Growth 45 (1978) 446-453). In this case, the substrate is silicon, so there are no problems with heat-resistance and matching of thermal expansion. Also, the ingredient is inexpensive so there is no restriction on resources, and products may be provided inexpensively. Further, the grown polycrystalline silicon film takes on the crystalline properties of the substrate, so poly crystals with good quality can be grown more readily than with cases using glassy carbon or ceramics for the substrate.
However, there are many problems in manufacturing, such as in the event of growing with the above-described CVD method, the ingredient gas cannot be used 100% effectively for growing, and growing silicon to a thickness of several dozen μm necessary for a solar cell requires the growth apparatus to be maintained frequently.
The liquid-phase growth method can be used for growing polycrystalline silicon. With the liquid-phase growth method, an ingredient such as silicon or the like is dissolved in a low-melting-point metal such as indium, gallium, tin, copper, aluminum, or the like, and adjusting a melt, following which supersaturation is induced by cooling or the like, so as to grow crystals on a substrate dipped therein. The silicon does not flow out from the melt, so there is no waste of the ingredients placed therein, and maintaining the apparatus is easy.
A method wherein high-quality polycrystalline silicon is grown on a polycrystalline substrate of inexpensive metallurgical grade silicon using this liquid-phase growth method, and forming a solar cell using the polycrystalline silicon, is disclosed in Japanese Patent Laid-Open No. 10-098205. According to this method, a polycrystalline substrate of metallurgical grade silicon can be filled in a mold and melted to achieve direct formation, so there is no need to take the trouble of ingot formation or slicing, and this method is further advantageous cost-wise. However, of course, substrates formed by forming ingots using inexpensive metallurgical grade silicon and slicing the ingot may be used suitably, as well.
However, as shown in FIG. 2, in the event of manufacturing solar cells using a polycrystalline growth layer 202 grown on a polycrystalline substrate 200, grid electrodes 206 formed on the surface thereof often exhibit line breaks in grid electrodes formed thereupon, regardless of whether metallurgical grade silicon is used or not. The grid electrodes 206 serve to collect carriers occurring due to incident light, and in the event that there are line breaks in the grid electrodes 206, the conversion efficiency of the solar cell deteriorates markedly. Also, forming the grid electrodes 206 so as to be heavier makes line breakage more difficult to occur, but loss of incident light due to the shadows of the grid electrodes 206 increases, which also is undesirable.
The cause of such problems is thought to be due to the following. A grain boundary 201 exists in the polycrystalline substrate 200, and the crystal orientation differs within crystal grains on either side of the grain boundary 201. Growing a polycrystalline growth layer 202 thereupon forms a grain boundary 203 at the same position as the grain boundary 201 on the substrate in the polycrystalline growth layer 202, since microscopically, crystals grow epitaxially at each of the regions of the crystal grains. A groove 205 is generally formed on the grain boundary 203, but as shown by K. J. Weber and A. W. Blakers, the groove tends to be deeper with poly crystals grown with liquid-phase growth as compared to poly crystals grown with CVD (J. Crys. Growth 154 (1995) 54-59). Accordingly, solar cells using such poly crystals readily exhibit line breaks in the grid electrodes 206, regardless of the excellent properties of the poly crystals grown with this liquid-phase growth.
On the other hand, increase in the thickness of poly crystals serving as the activation layer of the solar cell increases the output electric current due to increased absorption of incident light, which is a preferable property. However, this makes the groove deeper at the same time, making line breakage of the grid electrodes 206 more easy to occur.
Accordingly, it can be said that an average thickness W of the poly crystalline layer shown in FIG. 2 which is as great as possible, with an average depth G2 of the groove as small as possible, i.e., as small a G2/W as possible, serves as an indication of whether poly crystals are suitable for application to solar cells. In fact, as will be described later with reference to FIG. 1 in the description of the present invention, a small G1/W prevents grid electrode line breakage and increases output current thereby yielding high conversion efficiency.
G. Ballhorn, K. J. Weber, S. Armand, M. J. Stokes, and A. W. Blakers have proposed a method for reducing the depth of the groove formed at the grain boundary in liquid-phase growth of polycrystalline silicon on a polycrystalline substrate (Solar Energy Materials & Solar Cells 52 (1998) 61-68). As shown in FIG. 5, they state that the groove can be made less deep by, instead of simply dropping the temperature of the melt from Th to T1 during the period between dipping the substrate in the melt (t0) to pulling the substrate from the melt (tf), providing a period for heating the melt by ΔTh partway through the period. As for the reason thereof, they state that the grain boundary has an unstable structure and accordingly greater energy is required for growth thereupon as compared to growth within crystal grains, so the speed of growth relatively drops in the liquid grown method where growth occurs in a state close to thermal equilibrium in particular, thus forming a groove, but silicon melts out (melt-back) first from regions other than the grain boundary within the crystal grains upon being heated, so the groove becomes relatively shallower.
However, with large-scale liquid-phase growth apparatuses, there is the need to handle a large melt, anywhere from 10 kg or so up to several hundred kg. In the event of repeating cooling and heating of the melt as shown in FIG. 5, the time required for growth becomes longer, decreasing manufacturing throughput, and unnecessary energy is required for manufacturing, which is undesirable.